The goal of this research is to predict the changing metabolic heat production and chemical energy available for growth of neonatal mammals. The work begins with development of a mechanistic model of total energy balance which incorporates both the interdependencies of competing uses of chemical energy within the neonate and the important thermal interactions between nestlings and their mother. The next step involves mesurement of the critical physiological, behavioral and morphological parameters called for by the model for two species of small mammal. The final step will be to test the ability of the model to predict total litter metabolic rate and growth rate under experimental conditions.